FIG. 7 shows a conventional driving circuit corresponding to one pixel of a spontaneous light emitting type display device using an active matrix method which has been disclosed in the cited reference ‘T. P. Brody, et al., “A 6×6—in 20-1 pi Electroluminescent Display Panel”, IEEE Trans. on Electron Devices, Vol. ED-22, No. 9, pp. 739–748 (1975)”’. Tr1 denotes the first transistor which operates as a switching element. Tr2 denotes the second transistor which operates as a driving element for controlling the current of a spontaneous light emitting element. C1 denotes a capacitor connected to the drain terminal of the first transistor Tr1. A spontaneous light emitting element 60 is connected to the drain terminal of the second transistor Tr2. Next, an operation will be described. First of all, a voltage of a selection line 61 is applied to the gate terminal of the first transistor Tr1. At this time, when luminance data are applied at a predetermined voltage from a luminance data line 62 to a source terminal, a voltage level V1 corresponding to the magnitude of the luminance data is held in the capacitor C1 connected to the drain terminal of the first transistor Tr1. If the magnitude of the voltage level V1 held in the gate voltage of the second transistor Tr2 is enough for causing a drain current to flow, a current corresponding to the magnitude of the voltage level V1 flows from a voltage supply line 63 to the drain of the second transistor Tr2. The drain current becomes the current of the spontaneous light emitting element to emit a light.
FIG. 8 is a characteristic chart for explaining the generation of a variation in a luminance in the case in which the light emission is carried out in such an operation, showing the relationship between a voltage Vgs between a gate and a source of the second transistor Tr2 and the absolute value of a drain current Id. In the case in which it is impossible to produce FETs having the same characteristics over the whole display panel area due to manufacturing factors, for example, a variation shown in FIGS. 8(a), 8(b) and 8(c) is generated in threshold voltage Vt. When the voltage level V1 is applied between the gate and the source of the second transistor Tr2 having such characteristics A, B, and C, the magnitude of the drain current is varied from Id(a) to Id(c). Since the spontaneous light emitting element 60 shown in FIG. 7 emits light with a luminance corresponding to the magnitude of the current, a variation in the characteristic of the second transistor Tr2 causes a variation in a light emitting luminance in the spontaneous light emitting type display device.
FIG. 9 shows a driving circuit proposed to improve a variation in a light emitting luminance in the spontaneous light emitting type display device described above. The driving circuit has been disclosed in ‘R. M. A. Dawson, et al., “Design of an Improved Pixel for a Polysilicon Active—Matrix Organic LED Display”, SID 98DIGEST, 4. 2, pp. 11–14 (1998)’, corresponding to one pixel. FIG. 10 is a waveform diagram showing an operation timing based on the relationship between a time and an applied voltage in the driving circuit. In FIG. 9, reference numeral 1 denotes an organic electroluminescence element which is constituted by a light emitting material and two electrodes interposing the light emitting material and forms a pixel. Reference numeral 2 denotes a selection line for supplying a signal voltage for selecting a pixel over which a luminance control is to be carried out, reference numeral 3 denotes a luminance data line for supplying a voltage corresponding to a luminance, reference numeral 4 denotes the first transistor which is brought into a conduction state or a non-conduction state in response to a signal of the selection line 2, reference numerals 5 and 6 denote the first and the second capacitors for holding a voltage corresponding to the signal voltage component of the luminance data line 3, reference numeral 7 denotes the second transistor for controlling the current value of the organic electroluminescence element 1 corresponding to an electric potential difference Vgs on a point g to a point s, reference numeral 8 denotes the third transistor for connecting or blocking points g and d, reference numeral 9 denotes the first control signal line for supplying a signal voltage for controlling the third transistor 8 into a conduction state or a non-conduction state, reference numeral 10 denotes the fourth transistor for connecting or blocking the organic electroluminescence element 1 and the second transistor 7, and reference numeral 11 denotes the second control signal line for supplying a signal voltage for controlling the fourth transistor 10 into a conduction state or a non-conduction state. Reference numeral 12 denotes a voltage supply line for supplying a voltage to the organic electroluminescence element 1, and reference numeral 13 denotes a ground. The above-mentioned first to fourth transistors are P channel type FETs.
Next, an operation will be described. In the case in which all the first to fourth transistors in FIG. 9 are the P channel FETs, a positive voltage is applied to the voltage supply line 12 and each voltage shown in FIG. 10 is given to the luminance data line 3, the first control signal line 9, the second control signal line 11, and the selection line 2. First of all, the first transistor 4 is conducted at a time t1 and a pixel constituted by the organic electroluminescence element 1 is selected. At this time, the electric potential of the luminance data line is V0 corresponding to a luminance of zero. At a time t2, the transistor 8 is conducted so that the electric potential difference Vgs on the point g with respect to the point s has a smaller value than a threshold voltage Vt (a negative value) of the second transistor 7. At this time, a current flows to the organic electroluminescence element 1. When the fourth transistor 10 is brought into a non-conduction state at a time t3, electric charges of the capacitor 6 are discharged through the third transistor 8 until the Vgs reaches the threshold voltage Vt of the second transistor 7. At a time t4, the third transistor 8 is brought into a non-conduction state to hold the state of Vgs=Vt by the electric charges of the capacitor.
Next, when the voltage of the luminance data line 3 is changed by a luminance data voltage (a negative value), that is, is decreased to V0+[luminance data voltage] at a time t5, the Vgs is set to a voltage of Vs+Vt obtained by adding the voltage Vs (a negative value) which is proportional to the luminance data voltage and the threshold voltage Vt of the second transistor 7. The first transistor 4 is brought into a non-conduction state at a time t6 and the supply of the luminance data voltage is stopped at a time t7, thereby holding a state of Vgs=Vs+Vt. As shown in the equation, the second transistor 7 is operated as if the threshold Vt of the second transistor 7 becomes zero equivalently to the Vs at this time. In a series of processes, luminance data are written. When the transistor 10 is conducted in this state at a time t8, a current corresponding to the Vs flows to the organic electroluminescence element 1, thereby emitting a light. The light emitting state is maintained until a next data writing operation is carried out. This circuit can independently compensate for the threshold voltage of the second transistor 7 for controlling the current, that is, the luminance of the organic electroluminescence element 1 in each pixel. Therefore, there is an advantage that it is possible to suppress a variation in the luminance caused by a variation in the threshold voltage Vt in the second transistor 7 which controls each pixel.
The driving circuit according to the conventional example shown in FIG. 9 can eliminate the influence of the variation in the threshold voltage Vt in the second transistor 7 corresponding to each pixel on the precision in a luminance, that is, relationship between luminance data and the luminance of the organic electroluminescence element 1. As described in the explanation of the operation, the current flows to the organic electroluminescence element 1 for a period in which the third transistor 8 is brought into the conduction state at the time t2 in FIG. 10 so that the Vgs is set to have a smaller value than the threshold. Furthermore, when the fourth transistor 10 is then brought into the non-conduction state at the time t3, the voltage of the second control signal line 11 is changed. Since the gate electrode of the fourth transistor 10 has a capacitor component, a charging current flows to the capacitor component through the organic electroluminescence element 1. Since the two electrodes interposing the light emitting material of the organic electroluminescence element 1 inevitably act as the electrodes of the capacitor, moreover, the electric charges stored therein flow as a discharging current to the light emitting material of the organic electroluminescence element 1 for the non-conduction period of the fourth transistor 10.
As described above, these currents are generated for a period in which a pixel is selected, and moreover from the time at which the third transistor 8 is brought into the conduction state (t2 in FIG. 10) to the time at which the fourth transistor 10 is brought into the non-conduction state (t3 in FIG. 10), and are noise currents which are not related to a luminance data signal. Consequently, there is a problem that unnecessary light emission is caused to deteriorate precision in a luminance.
The present invention has been made to solve the problem and has an object to provide a spontaneous light emitting type display device having a high precision in a luminance which can prevent the unnecessary light emission of the organic electroluminescence element 1 due to a noise current for the data writing period of each pixel.